A major cause of death and poor health in significant segments of the population in the United States and in many other areas of the world involve disease and insufficient function of the heart. The vitality of all tissues in the body depends upon a continual flow of blood at an adequate rate to permit efficient and satisfactory function of the organs. The heart is required to function at a relatively high level. The heart typically pumps 75 gallons of blood per hour when the body is at rest and is required to function at even higher rates during moderate or heavy levels of exertion and activity.
Interruption or interference with the continuous and efficient function of the heart can occur for a variety of reasons. The arteries of the heart may become diseased and obstructed with the result that the heart will either develop insufficient blood flow or blood flow will become terminated. This arteriosclerosis is the well-known coronary artery disease that is a leading killer of some segments of the population, especially men.
Diseased coronary arteries often provide restricted circulation and diminished blood flow with the result that the heart is unable to carry out its normal function as it is gradually starved for blood. The result is that the heart fails to contract as forcefully as necessary with the result that the entire body suffers from insufficient blood flow.
When one of the coronary arteries becomes plugged by a blood clot, that particular area of the heart served by the plugged coronary artery will be cut off from an adequate supply of blood and, if circulation is not immediately resumed, the muscle tissue of that particular area of the heart will become impaired or die.
Insufficient or terminated blood flow in certain areas of the heart can also have deleterious effect on other functions of the heart, including the conduction system of the heart. The conduction system of the heart is a group of structures within the heart that determine heart rate in response to influences from the nervous system as well as the chemical information carried to the heart from other organs of the body. The conduction system provides stimulating impulses to all parts of the myocardium in a coordinated fashion. The coordination of the impulses is important to ensure that the different sections of the heart act in coordination to pump blood throughout the body. Coordinated function of the heart contraction ensure delivery of an adequate supply of blood to the various organs as demands on the organs vary. The stimulations necessary for proper excitation of the myocardium need to be coordinated to ensure the heart contracts effectively to make the heart an effective fluid pump.
The conduction system in the heart depends upon a regular generation of a depolarization wave of adequate magnitude to cause the myocardium to contract in an orderly fashion to force blood through the body's veins and arteries. The proper function of the heart is dependent upon the ability of the heart to generate or start a depolarization wave at a particular location in the heart in order to ensure a proper contraction. This depolarization wave must be generated in a place, at a location and with a frequency which is responsive to the needs of the heart as well as the other functions of the body.
The depolarization wave of the heart is generated as a result of some unique characteristics of myocardial tissue. Depolarization occurs as cell tissue, either muscle or nerve cells, is stimulated. The stimulus is then transmitted to the next cell in a process which is called depolarization. Through this process, a depolarization wave can be generated in a mass of muscle tissue with the result that the muscle tissue responds to the stimulus. This response results in the familiar muscle function or heart beat of the heart.
It has been learned that the generation of this depolarization stimulus is a characteristic of the behavior of the cell membrane of individual cells of living tissue. Living cells selectively permit the passage of various substances such as nutrients, oxygen, waste products through the cell membrane. These substances move freely through the cell membrane in order to ensure the adequate nutrition of the cell and maintain the life function of the cell. While there is a free movement of substances through the cell membrane, the movement is by no means unrestricted. Certain essential substances are blocked by the cell membrane so that certain essential substances are not permitted to move outside of the cell.
Other substances are not permitted to move from outside of the cell to the interior of the cell with the result that there is a substantial selectivity which occurs at the cell membrane preventing movement of selected elements through or across the cell membrane depending upon the nature of the substance. As an example, this selective permeability at the cell membrane works to keep substances such as potassium inside the cell and, at the same time, keep sodium out of the cell. Examination of the function of these two elements has revealed that they are instrumental in the proper function of heart cells and, indeed, are probably the basis for the generation of a depolarization wave necessary to stimulate heart beat in myocardial tissue.
When myocardial tissue cell is at rest, the concentration of sodium, which carries a positive electrical charge, on the outside of the cell membrane is about equal to the potassium concentration inside the cell membrane. When the cell is at rest and when the concentration of sodium and potassium on either side of the cell membrane are approximately equal, a balanced condition occurs in which essentially no activity occurs. In a normal cell, a relatively large number of substances carry a negative charge. These negatively charged substances are of a type which are retained within the cell by the selective permeability of the cell membrane. As a result, there normally are more negatively charged particles or substances inside the cell membrane than occur on the outside of the cell membrane. This results in a condition in which the inside of the cell membrane is more negatively charged than the outside of the cell membrane. This charge difference results in a voltage drop across the cell membrane that can be measured by sophisticated scientific equipment.
Measurement of the voltage drop across the cell membrane will reveal that the inside of the membrane is negatively charged with respect to the outside of the membrane. In this condition, the cell is polarized.
The polarized condition of the cell membrane normally exists uniformly throughout the cell membrane except when the cell membrane is disturbed or stimulated at a particular location. When a stimulant of selected types is applied to the cell membrane, the membrane loses its selective permeability at that particular site with the result that substances which normally are inhibited from moving across the cell membrane is lost. The cell membrane therefore no longer blocks the entrance of such a substance across the cell membrane. In such instances, the cell membrane does not block the entrance of sodium located outside of the cell. Sodium can then move to the interior of the cell. The stimulus capable of causing the cell membrane to lose its selective permeability characteristic can include a stimulus such as electrical, mechanical or thermal.
It has been found that, when the cell membrane is stimulated, sodium at the stimulated site rushes through and across the cell membrane flooding the interior of the cell with sodium at that particular location. This inrush of sodium further disturbs the membrane adjacent the original site of stimulation so that these adjacent areas of membrane also lose the property of selective permeability with the result that additional sodium is admitted over a wider area of the cell membrane. This progressively enlarged disturbance of the cell membrane expands outwardly from the original site of stimulation with the result that sodium enters in a wider and wider area in what appears to be an expanding wave front of stimulation originating from the original site of stimulation.
Finally, this progressive increase in the stimulation and the progressive loss of selective permeability progresses down the entire length of the cell to the end of the cell at which point the interior of the cell is flooded with sodium. Since sodium carries a positive charge, the negative charge on the cell interior is essentially neutralized so that the cell is then said to be depolarized. This depolarization upsets the steady state or relaxed condition of the cell interior and affects the myofibrals of the cell. Myofibrals are the strings of protein running the length of the cell. The depolarization of the interior of the cell causes the myofibrals to shorten with the result that the entire cell contracts or shortens in response to the depolarization.
As the cell depolarizes, potassium moves through the cell membrane to the outside of the cell. While the potassium exits through the cell membrane, the cell membrane also begins to pump sodium out of the cell. As the positively charged particles of sodium leave, the inside of the cell membrane again starts to become negative again. This reestablishment of the normal negative state in the interior of the cell continues until the original state of the cell is restored and the inside of the cell is again negative with respect to the outside of the cell. When this restoration of the negative condition of the cell occurs, the cell begins to relax and becomes repolarized. Thus, it is apparent that the cell in a repetition of this process undergoes a cycle of contraction and relaxation or depolarization and repolarization as a normal function.
This cycle occurs not in just one of the cells but occurs in all of the cells which are neighbors of the originally stimulated cell. A stimulated cell will pass on the depolarization to its neighbor cells with the result that a depolarization wave will radiate from an original site of stimulation, be it electrical, chemical or mechanical, in a wave pattern throughout a muscle group such as that represented by the heart. This depolarization wave creates a contraction of the muscle tissue in a wave pattern so that the pattern moves through the cells of the myocardium in a uniform and progressive manner, thus generating the pumping action which is characteristic of a healthy heart.
It was this characteristic of the heart muscle to react to electrical and mechanical stimulation which led A. S. Hyman to develop a machine for ambulance use in the 1930's which could be used to stimulate heart beat in accident victims. Hyman is credited with stating the principles of pacing through the use of small electrical stimulus applied to a relatively small area to give a rise to a contraction wave which spreads throughout the entire heart muscle giving the heart a relatively normal contraction. The device which Hyman developed included an electrode needle which could be passed through the ribs of a patient and into the heart for applying the electrical stimulus.
Later clinical work undertaken by Zoell advanced the understanding of the pacing process. In 1952, he used skin electrodes applied to the patient's chest to transmit an electrical shock to the heart causing it to contract. This work and other work proved to have lifesaving value and sparked interest in investigating the use of pacing. The result of this work has evolved to the point where, today, electro-mechanical pacemaker devices are routinely implanted in the muscle of the heart in order to apply a regular stimulus to the heart to set up regular heart beat in those patients having inadequate heart pacing function and thus ensuring better function of the heart as a fluid pumping mechanism.
The currently used electronic pacemakers are typically battery powered with the batteries having an average longevity of about five years. Typically, the electro-mechanical pacemaker is implanted in the heart by positioning an electrode in the apex of the right ventricle and the remainder of the pacemaker, including the battery, is implanted under the skin of the patient's chest. This operation is a relatively expensive implant procedure which is satisfactory for the life of the batteries used in the device. When the batteries are exhausted, the battery cannot be replaced and, in the typical patient, the entire pulse generator must be replaced. The cost of the replacement of the pacemaker is nearly the same as the original cost of implant.
As with any mechanical and electrical device, there are a number of problems which might be encountered with the device which will require remedial surgery. Batteries, as an example, might prematurely fail. Further, the generator may fail to provide proper electrical pulses to stimulate the heart muscle as needed. The typical electro-mechanical pacemaker includes sophisticated microelectronics which can prematurely fail. Further, the hermetically sealed pulse generator can develop a leak which will result in a short of the electrical circuitry necessary to the function of the generator.
The electro-mechanical pacemaker also employs a long flexible lead which extends from the pacemaker through the heart to the site of the implant of the electrode used to stimulate the heart muscle. This lead is subject to constant flexing with the risk that the flexing will ultimately result in a break in the lead.
Mechanical pacemakers also depend upon the success with which the electrode remains implanted in the right ventricle to stimulate the myocardial tissue. If the electrode should become disengaged, it would immediately result in misfunction or failure of the function of the pacemaker.
The need to implant the electrode in the heart in a secure fashion also includes a further problem of ensuring that the electrode remains in place without causing inflammation or rejection by the body tissue. Typically, the body will attempt to reject any foreign tissue or material which is imbedded in the tissues of the body. This rejection reaction can produce a systemic rejection process which will require removal of the electrode. Further, the imbedded electrode always presents the risk of infection in the muscle tissue with the potential for causing serious, if not fatal, trauma to the heart muscle.
A further disadvantage of the currently employed electro-mechanical pacemaker is the lack of an effective mechanism for detecting changes in demand for oxygen by the organs of the body. These electro-mechanical pacemakers stimulate the heart and generate the depolarization wave at a predetermined rate. This rate does not increase in response to increased demand by the organs of the body in the way natural pacing changes in the heart. Consequently, electro-mechanical pacemakers tend to place limitations on the level of physical activities of the user.
Accordingly, while great advances have been made in the pacing process employing effective mechanical processors, the use of such electro-mechanical pacers nevertheless pose substantial risks and disadvantages for the patient.